From what I can guess you have right clicked the material and clicked update thumbnail. It seems that when you update the thumbnail of a terrain material (detail material) it updates towards the color instead of the diffuse. If you double click on the material so you get it's properties you should see two fields called;
diffuseMap environment_levels\texturepaint\arena_kraken\arena_terrain_mud03.xbm
normalMap environment_levels\texturepaint\arena_kraken\arena_terrain_mud03_n.xbm
If you still have those there the material will still work, it's just that the thumbnail has become a flat color.
This should be reported as a bug, so that CD Projekt can fix it.
